In general, an image sensor is a device of displaying images by using a feature that a semiconductor reacts to light. That is, the image sensor reads out electrical values by detecting lightness and wavelengths of different lights derived from different objects. The image sensor makes the electric values to levels able to be signal-processed.
The image sensor is a semiconductor device of changing an optical image to electrical values. A charge-coupled device (CCD) is a device in which MOS capacitors are located very close to one another and charge, carriers are stored at the capacitors. On the other hand, a Complementary Metal Oxide Semiconductor (CMOS) image sensor uses CMOS technology which utilizes a control circuit and a signal processing circuit as peripheral circuits, employs MOS transistors as many as the number of pixels and adopts a switching scheme of detecting outputs sequentially by using the MOS transistors. The CMOS image sensor is used in personal portable systems such as a mobile phone since it consumes low power. Therefore, the image sensors are used in various areas of PC camera, medical instrument, toy and so on.
FIG. 1 is a block diagram showing a conventional image sensor.
Referring to FIG. 1, the image sensor includes a controlling & external system interfacing block 10, a pixel array 11, an analog-line buffering block 12, a column decoder 13, an analog bus 14, an analog signal processor (ASP) 15 and a digital signal processor (DSP) 16.
The ASP 15 contains a variable amplifier and an analog-to-digital converter (ADC) and the DSP 16 performs functions of error correction, color interpolation, gamma correction, color space conversion and so on.
Hereinafter, an operation of each component constructing the above image sensor will be explained in detail.
The pixel array 11 contains N×M pixels for maximizing a feature of reacting to light, N and M being integers, respectively, and is a core part of the image sensor for detecting information about images provided from the outside.
The controlling & external system interfacing block 10 controls a whole operation of the image sensor by using a finite state machine (FSM) and performs an interfacing operation for an external system. Since the block 10 includes an arrangement register (not shown), it is possible to program information related to various internal operations and the whole operation of the image sensor is controlled by the programmed information.
The analog-line buffering block 12 detects voltages of pixels on a selected one row and stores the detected voltages therein. Among the analog data stored in the analog-line buffering block 12, a data on a column chosen by the column decoder 13 is transferred to the variable amplifier in the ASP 15 through the analog bus 14.
The variable amplifier, e.g., a programmable gain amplifier (PGA), amplifies pixel voltages stored in the analog-line buffering block 12 when the pixel voltages are low. The analog data amplified at the variable amplifier are color-corrected and then converted to digital values at the ADC. The digital values are converted again according to video standards such as 4:2:2 or 4:4:4 at the DSP 16 after being processed by the above procedure.
Meanwhile, in the image sensor, a fixed pattern noise can occur by an offset voltage due to a minute difference in a manufacturing process. In order to compensate this fixed pattern noise, the image sensor adopts a correlated double sampling (CDS) scheme to read a reset voltage signal and a data voltage signal from each pixel of the pixel array 11 and output a difference between the reset voltage signal and the data voltage signal.
Application of the image sensor is still-camera, personal computer (PC) camera, medical, toy, and portable terminal and so on. If light sources are different, there occurs a flicker noise. Since, however, the greater part of application requires an image sensor usable without regard to a kind of light source, the image sensor needs a function of automatically eliminating the flicker noise.
The CMOS image sensor captures images by adjusting its exposure time to light. Therefore, if the exposure time is integer times of a frequency of a light source, there is no problem. However, if the exposure time is different from the frequency of the light source, in case of the CMOS image sensor capturing image data on a line-by-line basis, since the amount of light that each line receives is different, there occurs a flicker noise making a noise bar on a final image.
FIGS. 2A to 2E show graphs illustrating various cases in which flicker effects happen.
Referring to FIG. 2A, there is described the variation of light energy according to the change of time in case that the amount of inputted light, i.e., energy, has an identical frequency to that of the exposure time of the image sensor. In this case, since the amount of energy that each line receives is identical, a normal image is outputted.
FIG. 2B shows the energy having a different frequency from the exposure time, i.e., a case that the exposure time is shorter than a frequency of the light source. In other words, it shows a case that the frequency of the energy is longer than the exposure time, i.e., the energy changes in a shorter time period.
FIG. 2C shows a case that the frequency of the energy is shorter than the exposure time, i.e., the energy changes in a longer time period. In any case, there occurs flicker in the image. Specially, when there occurs a flicker noise, a noise bar generated on the image flows upward or downward, or the noise bar is fixed in the image. At this time, the fixed flicker noise is difficult to be distinguished with a real image.
FIGS. 2D and 2E represent cases that there occur flicker noises fixed in the image. In these cases, sizes of flicker noises occurring in a first frame and a second frame are identical regardless of the relation between the exposure time and the frequency of the light source. In case that the fixed flicker noises are generated, if detecting the flicker noises by using a flicker noise of a different size for each frame as in an existing algorithm, it is impossible to detect the flicker noises.
Therefore, as afore-mentioned, there needs to eliminate flicker for every case that the flicker occurs.